Numerous processes for producing sulfur dioxide have been proposed heretofore, these processes involved a roasting of sulfur-containing ores, particularly pyrites, or combustion of elemental sulfur. The processes may be described by the formulae: EQU S + O.sub.2 .fwdarw. SO.sub.2 and EQU 2MeS + 3 O.sub.2 .fwdarw. 2MeO + 2SO.sub.2
in the stoichiometry of this process, one mole of oxygen reacts with one mole of sulfur to yield one mole of sulfur dioxide. The ore-roasting furnaces which have been used heretofore are multiple-hearth furnaces, fluidized-bed furnaces and rotary kilns. The systems for burning elemental sulfur are generally fluidized-bed furnaces and especially atomizing burners of verious types.
For the most part, roasting and combustion are controlled so that the sulfur is roasted or burned as completely as possible to form gases which, to the greatest possible extent, are constituted by sulfur dioxide. In order to achieve this object, it is known to provide a sulfur-burning furnace with compressed air via a horizontal heating plate which forces the gas to flow along a predetermined path and prevents an escape of sulfur dust or fumes. It is also conventional to introduce sulfur power into a stream of air or oxygen in such manner that the sulfur is ignited at the moment it meets the stream of air or oxygen and is burned immediately.
In substantially all prior-art processes, an important problem is the formation of sulfur trioxide under the stringent conditions at which the sulfur is burnt or reacted. It has long been sought to provide a sulfur-combustion process which produces a gas free from sulfur trioxide or at least containing substantially less sulfur trioxide than is present in the ordinary sulfur-dioxide gases. The problem has been solved heretofore by the use of the oxygen-containing gas in substantial stoichiometric deficiency. It had also been proposed that oxygen or air be passed through hot liquid sulfur or the sulfur burned in the presence of a surplus of elemental sulfur or sulfur fumes to reduce the temperature at which combustion takes place. Mention may also be made of processes which form sulfur-dioxide gases by the combustion of sulfur with the aim to convert all or part of the sulfur dioxide to sulfur trioxide for use in the production of sulfuric acid or analogous products. In such systems, the heat content of the combustion gases is in part recovered and the sulfur-dioxide gas may be catalytically reacted to form sulfur trioxide. To this end, the sulfur is burned with the aid of a partial stream of predried air, i.e. a stream containing oxygen gas in a stoichiometric deficiency below that required to react all of the sulfur. The resultant gas stream is cooled in the heat exchanger, diluted with more predried air and the entire stream supplied to a catalytic reaction plant.
In all of the earliest processes for the purposes indicated, only a small throughput was obtained. In the more recent proposals, the disadvantage of a small throughput is obviated, although the higher concentration of sulfur dioxide in the combustion gas requires that the combustion be supported by oxygen-enriched air or pure oxygen, yielding in a higher combustion temperature, and gives rise to the formation, at the elevated temperatures, of nitrous oxide by reaction of the major components (N.sub.2 and O.sub.2) of ambient air. The nitrous oxide may result in a contamination of the end product, whether this consists of liquid sulfur dioxide or sulfuric acid, and also can lead to difficulties with respect to corrosion. It is not a practical solution to remove the nitrogen oxide from the product.